1. Field of the Invention
The present invention relates to off-the-road vehicles and in particular to an environmental control system for an enclosed cab of an off-the-road land vehicle.
2. Description of the Prior Art
In off-the-road vehicles such as agricultural tractors, it has become increasingly common to provide an enclosed cab to protect the operator of the vehicle from dust, noise and extremely hot or cold weather. Heating, ventilating and air condition (HVAC) apparatus has been developed to control the environment within the cab of the tractor. Examples of apparatus of this type are shown in U.S. Pat. Nos. 3,522,839 by Wendt et al; 3,776,358 by Williams; 3,868,896 by Doll et al; 4,072,487 by Irwin; and 4,120,527 by Lawrence.
It has been recognized in the past that it is desirable to pressurize the cab of an agricultural tractor to prevent dust and dirt from entering the cab. If dust and dirt are permitted to enter the cab and the HVAC apparatus, several detrimental effects can occur. First, the presence of airborne dust in the cab can make the farmer uncomfortable. Second, the dust is sucked into the air conditioner coil where it accumulates in the condensed moisture on the evaporator coil. The dust will continue to accumulate and eventually plug the air conditioner coil, thus preventing flow of air. Third, the dust will get into controls, switches, gauges, linkages and the like within the cab. The accumulation of dust can eventually adversely affect the operation of this equipment. In order to prevent dust and dirt from entering the HVAC system, it is typical to provide air filters which filter the fresh air being drawn into the apparatus. As dust and dirt accumulate in the filter, the fresh air filter becomes restricted, and the effectiveness of the HVAC system decreases. As a result, the environment within the cab becomes quite uncomfortable for the farmer. Often the farmer then opens the windows of the cab in an attempt to compensate for the loss of effectiveness of the HVAC SYSTEM. This permits dust to infiltrate the cab, and destroys the controlled environment of the cab.
In the past, it has been typical to use a single blower to draw in fresh air and to recirculate air within the cab. As a result, the blower is drawing air from two different sources: fresh air through a fresh air filter and recirculated air from the cab. As the fresh air filter begins to restrict, which in the case of the dusty and dirty environment occurs rapidly, the amount of air drawn from the fresh air intake drops off drastically, and the bulk of the air movement is recirculated air. This is because when the blower has more than one source of air, it will draw more air from the source which produces the least amount of resistance to air movement. As the fresh air filter restricts, therefore, fresh air intake is reduced, and ultimately cab pressure is lost.